The invention relates to a high-voltage vacuum-tube, specifically an X-ray tube, which comprises an electrode disposed in its vacuum space, which electrode, in the operating condition, carries a positive high voltage relative to an electrically conductive part by which it is at least partly enclosed. The electrode or a part connected to the electrode is connected to the conductive part via an insulator which is constructed so that in the operating condition the electrons which impinge over at least a substantial part of its surface area, encounters an electric field which field repels said electrons away from the insulator surface.
Such a high-voltage vacuum-tube is known from DE-OS No. 25 06 841 corresponding to U.S. Pat. No. 4,053,802. The electrode is then generally the anode of the high-voltage vacuum-tube. In the case of rotary-anode X-ray tunbes the electrode may be the shaft carrying the anode disc, which shaft is at the same potential as the anode disc. In general the electrically conductive part is the metal tube envelope of such a tube or a part thereof. Alternatively, in the case of rotary anode X-ray tubes, it may be a metal cylinder, which rotates together with an insulator and the shaft of the rotary-anode disc and which is connected to the housing of the X-ray tube via a bearing, as is known from DE-PS No. 24 55 974 corresponding to U.S. Pat. No. 4,024,424. Usually the insulator is constructed so that its frusto-conical inner jacket widens from the location where it is connected to the electrode in the axial direction.
In the known high-voltage vacuum-tube the shape of the insulator precludes insulator-surface flashover, which could impair the reliability of operation of the tube. However, in tubes which are subject to a substantial thermal load the increased temperature of the insulator reduces the binding energy of gaseous layers adsorbed at the surface, so that electron-stimulated desorption may increase and may rise to flash-over effects (R. A. Anderson, J. P. Brainard: Mechanism of pulsed surface flashover involving electron-stimulated desorption, J. Appl. Phys. 51, 1414, (1980)).